Experimental data are presented on the effect of inert (He, N2, Ar, CO2) and reactive (SH4, O2, CF3Br, (CH3O)3PO) gas additives to hydrogen flow and ambient air on flame separation during diffusion combustion of a hydrogen microjet emanating from a round micronozzle into air. The micronozzle is a stainless steel tube with an inner diameter of 0.5 mm, a length of 30 mm, and a wall thickness of 0.1 mm. The shadow method is used to determine the critical hydrogen velocity at which the flame of the hydrogen microjet separates from the nozzle upon injection of additives into air and into the hydrogen flow. It is found that the addition of the above gases to hydrogen narrows the microjet velocity range in which stabilization of the flame can be achieved with injection of both inert and reactive additives. For the hydrogen flow with the additives studied, the critical velocity of flame separation from the micronozzle depends primarily on the mean molecular weight of the gas mixture. In the case of the addition of the gases to the air surrounding the diffusion flame, the critical velocity of flame separation is determined by their effect on the chemical reactions of hydrogen oxidation (inhibiting effect) and by the reduction of oxygen concentration due to air dilution with the additive. The results could be of interest to experts in hydrogen energetics for determining the conditions of stable hydrogen microjet combustion and the minimum concentrations of flame-suppressing additives to air preventing the ignition and combustion of hydrogen after its accidental release.